A few must-know organic reaction mechanisms for chemistry majors (1)
Reaction Mechanism
The peracetic acid first undergoes nucleophilic addition to the carbonyl group, and then one of the hydrocarbon groups on the keto carbonyl group migrates with a pair of electrons to the oxygen atom in the -O-O- group that is directly attached to the carbonyl carbon atom, and at the same time, the O-O bond isisolated. Thus, this is a rearrangement reaction
In the reaction of optically active 3–phenylbutanone with peracetic acid, the berry pattern of the chiral carbon atom of the rearrangement product remains unchanged, indicating that the reaction is an intramolecular rearrangement:
Asymmetric ketone oxidation, in the rearrangement step, both groups can migrate, but still have a certain selectivity, according to the ability to migrate its order:
The mechanism for oxidation of aldehydes is similar, but the migration is of hydrogen-negative ions to give carboxylic acids.
Reaction examples
Oxidation of ketones with peroxy acids such as peroxy, peroxybenzoic acid, m-chloroperoxybenzoic acid, or trifluoroperoxy can produce the corresponding ester by inserting an oxygen atom next to the carbonyl group, with trifluoroperoxy being a very good oxidizing agent. These oxidizing agents are characterized by fast reaction rates, reaction temperatures generally between 10 and 40°C, and high yields.
Beckmann rearrangement reaction
Oximes under the action of acid, polyphosphoric acid, as well as phosphorus pentachloride, phosphorus trichloride, benzenesulfonyl chloride, thionyl chloride, etc. can generate the corresponding substitution of amides, such as cyclohexanone oxime under the action of the rearrangement of the generation of caprolactam:
Reaction mechanism
Under the action of acid, the oxime first undergoes protonation, and then removes one molecule of water, while the group in the antiposition with the hydroxyl group migrates to the electron-deficient nitrogen atom, and the carbon positive ion formed reacts with water to obtain the amide.
If the migrating group is a chiral carbon atom, its configuration remains unchanged before and after migration, for example:
Examples of reactions
Cannizzaro reaction
When an aldehyde with no active hydrogen in the α-position carbon atom reacts with a concentrated NaOH or KOH aqueous or alcoholic solution, it disproportionates to produce a mixture of an acid (salt) and an alcohol comparable to the aldehyde without alcohol-formaldehyde condensation or resinification. This reaction is characterized by the oxidation and reduction of the aldehyde itself, one molecule is oxidized to the salt of the acid, and the other molecule is reduced to an alcohol:
Fatty aldehydes, only formaldehyde and the carbonyl group is connected to a tertiary carbon atom aldehydes, this reaction occurs, other aldehydes and strong bases, the role of alcohol-aldehyde condensation or further into a resin-like material.
Aldehydes with α-active hydrogen atoms and formaldehyde firstly undergo hydroxyl aldehyde condensation reaction to get β-hydroxy aldehyde without α-active hydrogen atoms, and then cross Cannizzaro reaction with formaldehyde, such as acetaldehyde and formaldehyde reaction to get pentaerythritol:
Reaction Mechanism
The aldehyde first undergoes nucleophilic addition with the hydroxide anion to obtain the anion, and then the hydrogen on the carbon carries a pair of electrons in the form of a hydrogen-negative ion that is transferred to the carbonyl group of another molecule that cannot be carbonized.
Claisen ester condensation reaction
Esters containing α-hydrogen undergo condensation in the presence of a basic condenser such as sodium alcohol, losing one molecule of alcohol to give a β-keto acid ester. If 2 molecules are condensed under sodium metal and a small amount to get acetyl.
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